Wedge-wedge bonding is a generally recognized and reliable technique for establishing electrical contact between a chip and a substrate. It is used both in power electronics and in microelectronics. To monitor the quality of the bonding process, a specific percentage of the products are taken during the course of production and subjected to a destructive test, and the quality of the overall production batch is assessed using statistical methods.
The desire for further quality control, in particular in areas where safety is an issue, has led to the development in recent years of several methods that allow in-line monitoring to be performed in addition to the destructive test specified above. Broadly speaking, two methods should be mentioned, on the one hand the nondestructive mechanical testing of a bonded connection, in which low forces act on the connection (tensile or shearing forces), in order to test its strength, on the other hand a method based on the measurement of signals that can be obtained directly from the ultrasonic generator or the position sensor of the transducer.
The second method uses the current flowing in the transducer, the voltage at the output of the ultrasonic generator or the deformation of the wire, or a combination of these as measured variables.
The bonding force, the ultrasonic power and the bonding time serve as correcting variables for defining a process window. These methods are described in the U.S. Pat. No. 4,606,490, EP 0 368 533 B1 and EP 0 540 189 B1. An essential feature of these methods is that the variables measured vary with a profile that follows a characteristic curve predetermined by the user. A weakness of this method is that even poor bonded connections may exhibit such deformation and current profiles. Consequently, the set procedure specified is necessary, but in no way sufficient for determining the quality of a bond, and is therefore unsuitable for detecting with certainty a bond of inferior quality.
An attempt to rectify this shortcoming can be found in the patents EP 1 023 139 B1 and EP 1 342 201. In the earlier methods, only one measured variable was considered and taken as a basis as a measure of quality. In recent methods, it is attempted to combine the aforementioned variables and derive quality criteria from them. Here, instead of the current, the electrical admittance of the transducer is used as a variable that is most independent of the ultrasonic power. It is attempted to use the combination of the admittance profile and the deformation of the wire, and the correlation between them, to achieve a more reliable finding as to the quality of a connection.
These methods do not achieve the aim either, since the measured variables used do not correlate sufficiently with the physical processes taking place in the connecting zone. For example, the electrical resonance (phase angle between current and voltage=zero) does not correspond to the mechanical resonance (phase angle between the speed of the tip of the tool and the voltage=zero), so that systems which only use the electrical variables are not operated with mechanical resonance. Neither the speed of the tip of the tool nor the friction between the parts being connected, that is to say the physical variables that are of decisive importance directly for the formation of the connection, are recorded by the aforementioned methods.
The use of a sensor mounted in the membrane is described in the applicant's EP 1 789 226 A1 and WO 2006/032316 A1.
The described methods that have so far become known are therefore not suitable for acquiring the data essential for assessing the quality of a connection and processing them correspondingly. To this extent, these methods cannot satisfy the requirements for quality monitoring in fully automatic bonders. It is therefore desired to provide a method and apparatus for quality control during wire bonding operations that addresses these deficiencies.